Osteosarcoma (OS) is one of the most common primary bone cancers and often occurs in either young people in their 10s and 20s or elderly adults in their 60s and 70s. Our understanding of the genetic factors critical for OS formation and metastasis is still limited. Lack of understanding for the genetic pathways of those factors impedes the progress of developing effective therapeutic strategies to treat this malignant disease. The p53 gene, which is mutated in more than 50% of human tumors, has been reported in sporadic OS. Recently, our and other studies suggest that activation of Notch signaling contributes to the pathogenesis of human OS and its inhibition may be a therapeutic approach for the treatment of OS. Notch signaling plays an important role in developmental processes and adult tissue homeostasis by regulating cell fate determination, proliferation, differentiation and apoptosis. Altered Notch signaling has been associated with several cancers and can act both as an oncogene and tumor suppressor gene depending on its expression levels and timing. In this project, we propose to study the genetic interaction between the Notch signaling and p53 pathways in the development of bone tumors. This will address an important interface between stress-generated p53 pathway and skeletal developmental pathway of Notch signaling in the pathogenesis of OS. By applying both Notch gain and loss of function models in vivo, we will determine its requirement and genetic interaction with p53 in formation of OS. Success of this proposal will have an important positive impact on clinical application, because it will identify a genetic role of Notch signaling in development of OS. The proposed Specific Aims provide direction in identifying a crosstalk of Notch signaling and p53 pathways for initiation and/or progression of OS. In addition, these studies will provide the genetic mechanistic frameworks of how Notch signaling and p53 interact in the context of bone cells. Eventually, understanding of those genetic pathways and their interaction will lead to and facilitate the development of novel approaches for therapies in humans.